LM27953TL datasheet(9/11 Pages) NSC

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Circuit Description (Continued)

When the voltage on the EN pin is low (<0.5V), the part is in

shutdown mode. All internal circuitry is OFF and the part

consumes very little supply current when the LM27953 is

shutdown. When the voltage on the EN pin is high (>1.1V),

the part is active. The charge pump is ON, and the corre-

sponding output current drivers are active.

EN is also used to turn the output currents ON and OFF.

SETTING LED CURRENTS

The output currents of the LM27953 can be set to a desired

value simply by connecting an appropriately sized resistor

SET) between the ISET pin of the LM27953 and GND. The

output currents (LED currents) are proportional to the current

that flows out of the I

SET pin. The output currents are a factor

of 100 greater than the I

SET current. The feedback loop of an

internal amplifier sets the voltage of the I

SET pin to 1.25V

(typ.). Placing a resistor between I

SET and GND programs

the I

SET current, and thus the LED currents. The statements

above are simplified in the equations below:

Dx =100x(VSET /RSET)

SET = 100 x (1.25V / IDx)

MAXIMUM OUTPUT CURRENT, MAXIMUM LED

VOLTAGE, MINIMUM INPUT VOLTAGE

The LM27953 can drive 4 LEDs at 15mA each from an input

voltage as low as 2.7V, so long as the LEDs have a forward

voltage of 3.5V or less (room temperature).

The statement above is a simple example of the LED drive

capabilities of the LM27953. The statement contains the key

application parameters that are required to validate an LED-

drive design using the LM27953: LED current (I

LEDx), num-

ber of active LEDs (N), LED forward voltage (V

LED), and

minimum input voltage (V

IN-MIN).

The equation below can be used to estimate the total output

current capability of the LM27953:

LED_MAX = ((1.5 x VIN)-VLED)/((NxROUT)+kHR) (eq. 1)

LED_MAX = ((1.5 x VIN )-VLED) / ((N x 2.7

Ω) + 22mV/mA)

OUT – Output resistance. This parameter models the inter-

nal losses of the charge pump that result in voltage droop at

the pump output P

OUT. Since the magnitude of the voltage

droop is proportional to the total output current of the charge

pump, the loss parameter is modeled as a resistance. The

output resistance of the LM27953 is typically 2.7

Ω (V

IN =

3.0V, T

A = 25˚C). In equation form:

POUT = 1.5xVIN – NxILEDxROUT

(eq. 2)

HR – Headroom constant. This parameter models the mini-

mum voltage required to be present across the current

sources for them to regulate properly. This minimum voltage

is proportional to the programmed LED current, so the con-

stant has units of mV/mA. The typical k

HR of the LM27953 is

22mV/mA. In equation form:

POUT –VLED) > kHRxILED

(eq. 3)

The "I

LED-MAX" equation (eq. 1) is obtained from combining

the R

OUT equation (eq. 2) with the kHR equation (eq. 3) and

solving for I

LED. Maximum LED current is highly dependent

on minimum input voltage and LED forward voltage. Output

current capability can be increased by raising the minimum

input voltage of the application, or by selecting an LED with

a lower forward voltage. Excessive power dissipation may

also limit output current capability of an application.

PARALLEL Dx OUTPUTS FOR INCREASED CURRENT

CAPABILITY

Outputs D1 through D4 may be connected together in any

combination to drive higher currents through fewer LEDs.

For example in Figure 1 , outputs D1A and D2A are con-

nected together to drive one LED. D3A and D4A are con-

nected to drive a second LED.

With this configuration, two parallel current sources of equal

value provide current to one of the LEDs. R

SET should

therefore be chosen so that the current through each output

is programmed to 50% of the desired current through the

parallel connected LED. For example, if 40mA is the desired

drive current for the parallel connected LED, R

SET should be

selected so that the current through each of the outputs is

20mA. Other combinations of parallel outputs may be imple-

mented in similar fashions, such as in Figure 2 .

Connecting outputs in parallel does not affect internal opera-

tion of the LM27953 and has no impact on the Electrical

Characteristics and limits previously presented. The avail-

able diode output current, maximum diode voltage, and all

other specifications provided in the Electrical Characteristics

table apply to parallel output configurations, just as they do

to the standard application circuit on pg1 of the datasheet.

SOFT START

The LM27953 contains internal soft-start circuitry to limit

input inrush currents when the part is enabled. Soft start is

implemented with a controlled turn-on of the internal voltage

reference. During soft start, the current through the LED

20128019

FIGURE 1. Two Parallel Connected LEDs

20128020

FIGURE 2. One Parallel Connected LED

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